Four helophyte species growing under salt stress: their salt of life

Four helophyte species growing under

salt stress: their salt of life?

Michiel J.M. Hootsmans*, Frank Wiegman

Department of Environmental Science and Water Resources, IHE-Delft, P.O. Box 3015,

2601 DA, Delft, The Netherlands

Received 26 September 1997; accepted 3 June 1998

Abstract

We studied the capacity of pre-germinated seedlings of four helophyte species to develop under

conditions of chronic and temporary salt stress in waterlogged sediments: Typha latifolia L.,

Phragmites australis (Cav.) Trin. ex Steud., Scirpus lacustris ssp. lacustris L., and Scirpus

maritimus L. Salinity levels were 1:8%S and 18%S, applied constantly during four months, or for

one or three weeks in otherwise freshwater conditions. The chronic salt stress treatment at 18% S

led to a rapid die-off of both T. latifolia and S. lacustris ssp. lacustris. The other two species only

barely survived, hardly forming new vertical shoots. At 1.8% S, the consequences were less, and

limited to T. latifolia and S. lacustris ssp. lacustris. Both levels of short-term salinity stress were

well-tolerated by P. australis and S. maritimus, while mortality of T. latifolia and S. lacustris ssp.

lacustris was significantly increased after the three weeks, 18% S treatment. The surviving

individuals of these two species after this treatment also showed a more than 50% reduction in their

total biomass compared to the control plants. Remarkably, after the three weeks 18% S treatment,

all species apart of T. latifolia showed a short period in which the growth rate of the number of new

shoots was significantly increased. This increase was sufficient to restore the same development

pattern as observed in the control plants. # 1998 Elsevier Science B.V. All rights reserved.

Keywords: Salt tolerance; Helophyte; Seedling development; Restoration

1. Introduction

Lake Volkerak-Zoom is a former marine tidal estuary, which came into existence in

1987 after the closure of several dams in the south-western part of The Netherlands. After

Aquatic Botany 62 (1998) 81±94

* Corresponding author. Tel.: +31 152 151 789; fax: +31 152 122 921; e-mail:[email protected];internet:http://www.ihe.nl

0304-3770/98/$ ± see front matter # 1998 Elsevier Science B.V. All rights reserved.

PII: S 0 3 0 4 - 3 7 7 0 ( 9 8 ) 0 0 0 8 5 - 0

its creation, the lake water quickly freshened, while saline groundwater remained in

deeper or shallower parts of the sediments. During the first years after the closure, the

lake was characterized by a very high transparency of the water and a diverse submerged

freshwater macrophyte community (Schutten et al., 1994). However, the development of

an extensive belt of shoreline vegetation strongly lagged behind.

Thus, spawning and nursery areas for important predatory fish species like Pike, Esox

lucius L., are scarce and this has probably contributed to a strong development of

whitefish like Bream, Abramis brama (L.) (Ligtvoet and De Jong, 1995). Together with

on-going eutrophication, this in turn may have caused the recent change of the system

from a desired, clearwater, macrophyte-dominated state to a turbid, algae-dominated state

(for a general discussion of these phenomena, see a.o. De Nie, 1987; Moss, 1990;

Scheffer et al., 1993; and Van Vierssen et al., 1994).

The managing authorities started a research project to evaluate options for action in the

lake (Ter Heerdt, 1995; Vulink and Coops, 1995). As a working hypothesis, it was

assumed that the very limited development of shoreline vegetation is mainly due to the

high amount of sea salt in the lake shore sediments, which is only slowly disappearing by

the flushing action of fresh surface water and rainwater. In the upper 10 cm of underwater

sediment, salt concentrations currently are relatively low (0.1±1.0 g Clÿ1 lÿ1 interstitial

water), rapidly increasing with depth to 5±8 g Clÿ1 lÿ1 at 50 cm. However, saline

groundwater in shoreline sediments that have been exposed to the air since the closure of

the estuary is easily transported upwards by capillary movement, leading to very high

concentrations (from 20 g Clÿ1 lÿ1 at 50 cm depth upto 140 g Clÿ1 lÿ1 in the top 2.5 cm).

The same salinization process was demonstrated in underwater sediment that was

exposed after experimental drawdown of the watertable (data taken from Slager, 1996).

The present study focused on the effects of chronic and temporary salt stress on growth

and development of helophytes under controlled conditions, growing from the seedling

stage. Four species, already present in the area of Lake Volkerak-Zoom, were used:Typha

latifolia L., Phragmites australis (Cav.) Trin. ex Steud., Scirpus lacustris ssp. lacustris L.,

and Scirpus maritimus L. Information on salinity tolerance in these species is scarce, and

appears limited to P. australis and S. maritimus (Mercado et al., 1971; Van der Toorn,

1972; Lissner and Schierup, 1997).

2. Materials and methods

2.1. Experimental design

The experiments were performed outdoors on an unshaded roof of the IHE-Delft

premises. In a randomized block design, seven treatments, differing in salinity level and

duration of the salt stress, were applied (see Table 1). Each of the 10 blocks used,

contained one replicate plastic container (60 cm�40 cm�40 cm) for each treatment.

Salinities were created using artificial sea salt and tapwater, but no nutrients were added

to the water and thus, differences in nutrient concentration between salinity treatments

were negligible. For comparisons, our (sea salt) salinity values can be divided by 1.8 to

convert them into chlorinity values.

82 M.J.M. Hootsmans, F. Wiegman / Aquatic Botany 62 (1998) 81±94

One individual of each of the four species of helophytes per container was present,

each in its own 5 l plastic bucket. This made competition between plants in one container

for the (low) nutrient supply in the outside water unlikely. The bottom of the buckets was

perforated with five holes of 5 mm diameter, and covered with a 2 cm layer of pebbles to

prevent sediment wash-out. A sandy sediment, coming from the littoral zone of Lake

Volkerak-Zoom, was used, and the water level inside and outside of the bucket was kept

at the sediment surface. Tapwater was added to replace evaporated water. To increase or

reduce the salinity level, the buckets were raised for a short time above the water to drain.

If necessary, the flushing was repeated several times to achieve the required salinity level

inside the buckets, which in general was reached within 2 days. Control buckets were not

flushed and thus, sediment redox conditions may have differed between these treatments

and the controls. As the nutrient content in the water surrounding the buckets was low

(tapwater levels), flushing of the temporary salt stress treatments could have led to a

(temporarily) reduced nutrient content of the porewater.

Our experiments started 18 May 1995 with seedlings less than one week old. Seedlings

were provided by RIZA Lelystad, using seeds collected in the Netherlands. Experiments

started on 18 May 1995. Some seedlings died within a few days after planting, and thus

some re-planting of newly germinated material was done in the first two weeks to

establish as many replicate plants as possible. The temporary salt stress was applied

between 19 July and 9 August (three week period) and between 26 July and 2 August

(one week period). The experiments were harvested in the period of 18±19 September.

2.2. Measurements

Photosynthetic photon fluence rate (PPFR, mmol mÿ2 sÿ1, 400±700 nm) was measured

using a LiCor 192 sb quantum sensor. Temperature of the air (inside a standard weather

station) and water temperature of one container were measured using thermistors. Half-

hour averages of temperature and light data were recorded with a Campbell 21x

datalogger. Salinity was measured as conductivity using a WTW salinity meter. Salinity

of the interstitial water in a number of replicate buckets per salt stress treatment was

regularly monitored using artificial roots (RHIZON SSS1). These are hollow, flexible

sticks (70 mm�3 mm) made from a semipermeable material, coupled to a syringe.

Table 1The treatments used in the experiment

Treatment No. Salinity (%) Duration of salt stress

1 0 No salt stress (freshwater control)

2 1.8 4 months

3 18 4 months

4 1.8 1 week, otherwise freshwater

5 18 1 week, otherwise freshwater

6 1.8 1 weeks, otherwise freshwater

7 18 1 weeks, otherwise freshwater

During the middle of the 4 month experimental period, the temporary salt stress treatments were applied: the 3weeks exposure lasted from 19 July till 9 August; the 1 week exposure was from 26 July till 2 August. Duringthe remaining period, the temporary salt stress treatments contained freshwater.

M.J.M. Hootsmans, F. Wiegman / Aquatic Botany 62 (1998) 81±94 83

After the first month of establishment, the following weekly data were obtained: length

of the main shoot (including uppermost leaves, in cm), total number of leaves of the main

shoot, and the number of new vertical shoots per replicate plant. At the end of the

experiment, dryweight (DW, 72 h, 708C) of aboveground and belowground biomass per

replicate clone was determined, and the number of new S. maritimus tubers was counted.

2.3. Analysis

In the case of the number of leaves and the length of the main shoot, results from a

replicate were removed from the dataset from the moment at which the main shoot

appeared to have died (defined as plants with a brownish, wilted appearance, showing no

change in leaf number or length). In the case of the number of new vertical shoots, this

was done from the moment at which the whole clone in the bucket was considered dead.

Aboveground biomass development during the experiment was estimated by a set of

linear regression equations with a forced zero intercept, describing the allometric relation

between morphological characteristics and aboveground biomass (in g DW) at the date of

harvesting. These relations were obtained by stepwise multiple regression per species

using the SAS statistical package (SAS Institute, 1988). Weekly linear relative growth

rates (RGR) of estimated aboveground biomass were calculated and plotted against the

mean date (in days) of the period for which the RGR estimate was made.

For statistical analysis, the mentioned SAS package was used. For all characteristics,

the results were analysed separately for the set of two chronic salt stress treatments and

the control, and for the four temporary salt stress treatments and the control. A one-way

analysis of variance (ANOVA) was applied to test treatment effects for each date, while a

two-way ANOVA tested the effect of age, treatment and their interaction. When a

treatment effect is stated as being significant, its significance level was at least p<0.05. If

the data were not normally distributed, or when variances were not homogeneous, data

values were transformed using a log(x�1) transformation. All multiple comparisons tests

per sampling date were done using student-t tests. Using the Bonferroni method, the

comparison-wise error level was kept sufficiently low to ensure an overall experiment-

wise error level of 5% for the complete set of comparisons per date. In the chronic salt

stress experiment, sometimes all plants of treatment 3 (18% S) died. In that case, the

values of the two remaining treatments 1 and 2 were compared with zero.

3. Results

3.1. Environmental conditions

During the experiment the irradiance values remained relatively constant, at about

37 mol mÿ2 dÿ1, with peak light intensity values of about 1300 mmol mÿ2 sÿ1 reached

around midday. Daily mean temperatures of air and water varied between 15 and 258C.

During the second half of June, quite some rainfall lowered the originally installed

interstitial water salinity levels by half for a two week period. Subsequently, July and

August had dry, sunny weather, with an average maximum temperature of about 258C.


Irradiance and temperature values during the temporary salt stress period are shown in

Table 2. There were no significant differences in irradiance and temperature between the

mean values in the first week and the mean values for the whole three weeks period. Thus,

it is concluded that the one week and three weeks temporary salt stress treatments did not

differ in irradiance and temperature.

In Fig. 1 mean salinity values are shown for the duration of each salt stress treatment,

both for the interstitial and the open water. Although significant differences between

desired and realized salinity levels were detected for some treatments, only the salinity of

the interstitial water in treatment 5 (short, one-week stress of 18% S) appeared

considerably less than desired. All other salinity levels were reached quite satisfactorily.

3.2. Effects of salt stress on mortality and final biomass

Mortality at the end of the experiment showed significant differences between the

species (c2-test) for treatments 3 and 7 (chronic and three weeks exposure to 18% S,

respectively). In treatment 3, all plants of T. latifolia and S. lacustris ssp. lacustris died,

Table 2Weekly averages of daily mean irradiance, peak light intensity and daily mean air and water temperature duringthe period of the two temporary salt stress treatments

Period 19±25 July 26 July±2 August 3±9 August Whole period

Parameter

Irradiance (mol mÿ2 dÿ1) 42.5�3.5 38.2�3.5 37.4�3.8 39.3�2.0

Peak light intensity (mmol mÿ2 sÿ1) 1436�115 1280�121 1302�127 1337�68

Air temperature (8C) 20.6�0.9 23.4�0.8 19.9�0.8 21.4�0.6

Water temperature (8C) 21.7�0.8 23.3�0.6 20.9�0.7 22.0�0.4

Data are means�one standard error.

Fig. 1. Mean salinity during the various salt stress treatments. Note: for the temporary salt stress treatments (4±

7), only data on salinity taken during the actual application of salt stress were used. Asterisks indicate salinities

which differ significantly (p<0.05, t-test) from the desired salinity level (indicated with horizontal lines: 1.8 and

18% S). In this and in following figures, error bars indicate one standard error.


while mortality was 60% in P. australis and S. maritimus. In treatment 7, T. latifolia had

70% mortality, S. lacustris ssp. lacustris showed 30% mortality, while all plants of the

other two species survived.

Besides these effects on mortality, differences were also found in the total biomass per

plant at the end of the treatments (Table 3). T. latifolia showed a significant decrease in

total biomass for treatment 2 (constant 1.8% S). The surviving individuals of P. australis

and S. maritimus in treatment 3 (constant 18% S) had a significantly depressed total

Table 3Mean results for living plants at the end of the 4-month experiment

Species Treatmenta Total Aboveground/

belowground

biomass (a/b)

Length Leafb No. of

new shootsc

(g DW potÿ1) (g gÿ1 DW) (cm)

T. latifolia 1 10.0�1.45 cB 0.38�0.04 bA 61.0�2.12 cC 7.2�0.20 cA 0.6�0.16 bA

2 4.9�0.58 b 0.45�0.03 b 46.5�1.41 b 6.4�0.16 b 0.1�0.10 a

3 0.0�0.00 a 0.00�0.00 a 0.0�0.00 a 0.0�0.00 a 0.0�0.00 a

4 8.7�0.91 AB 0.37�0.03 A 55.9�2.57 C 6.8�0.28 A 0.6�0.18 A

5 7.3�1.36 AB 0.45�0.05 AB 52.2�2.78 BC 7.2�0.32 A 0.7�0.29 A

6 8.3�1.06 AB 0.32�0.02 A 53.7�2.86 BC 6.7�0.26 A 0.2�0.13 A

7 2.8�0.79 A 0.66�0.20 B 39.5�4.37 B 6.7�0.33 A 0.0�0.00 A

S. lacustris 1 11.2�1.85 bB 0.21�0.02 bA 15.9�2.70 bA 1.1�0.14 bA 8.7�0.93 bA

ssp. lacustris 2 7.6�1.10 b 0.31�0.02 c 27.7�3.75 c 1.4�0.18 b 7.0�0.38 b

3 0.0�0.00 a 0.00�0.00 a 0.0�0.00 a 0.0�0.00 a 0.0�0.00 a

4 12.2�1.35 B 0.23�0.01 A 20.0�2.75 A 1.0�0.00 A 8.7�0.56 A

5 8.8�1.13 AB 0.32�0.03 BC 18.6�1.44 A 1.0�0.00 A 9.8�0.65 A

6 10.1�1.88 AB 0.21�0.01 A 20.1�2.47 A 1.3�0.25 A 7.9�0.77 A

7 3.9�0.73 A 0.43�0.02 C 16.0�2.44 A 1.0�0.00 A 7.0�0.90 A

P. australis 1 14.9�2.34 bA 0.34�0.03 aA 63.5�2.58 cA 9.2�0.42 bB 10.8�1.23 bA

2 10.7�1.25 b 0.36�0.02 a 51.8�2.53 b 9.0�0.37 ab 10.1�0.91 b

3 0.9�0.36 a 1.31�0.53 b 19.4�4.30 a 6.3�1.02 a 2.7�0.88 a

4 12.9�2.45 A 0.31�0.03 A 58.2�2.97 A 7.7�0.63 AB 9.4�0.98 A

5 15.9�1.45 A 0.33�0.02 A 57.3�3.70 A 7.7�0.62 AB 13.5�1.34 A

6 12.9�1.57 A 0.27�0.02 A 63.6�3.95 A 7.1�0.38 AB 8.4�1.25 A

7 14.1�2.26 A 0.34�0.03 A 52.2�4.57 A 6.7�0.67 A 12.7�1.57 A

S. maritimus 1 12.5�1.02 bA 0.14�0.01 aB 34.6�3.22 bA 9.3�0.45 bA 5.3�0.88 bA

2 10.8�0.93 b 0.12�0.01 a 29.4�1.82 b 9.4�0.45 b 4.4�0.37 b

3 0.4�0.11 a 0.20�0.02 b 10.0�1.04 a 2.0�0.32 a 0.7�0.49 a

4 15.0�1.36 A 0.12�0.00 AB 35.5�2.96 A 9.3�0.55 A 5.7�1.22 A

5 16.5�1.35 A 0.13�0.01 AB 40.3�2.17 A 10.3�0.42 A 5.6�0.70 A

6 12.4�2.04 A 0.10�0.01 A 31.0�2.30 A 8.6�0.50 A 4.2�0.65 A

7 15.2�2.08 A 0.14�0.01 B 33.4�2.18 A 8.9�0.60 A 8.1�1.62 A

aTreatments: 1 ± control; 2 ± 1.8% S; 3 ± 18% S (both during the whole experimental period); 4 ± 1.8% S;5 ± 18% S (both 1 week); 6 ± 1.8% S; 7 ± 18% S (both 3 weeks).bNumber of leaves of the main shoot.cEach�one standard error.Means with the same letter are not significantly different (multiple t-tests, experiment-wise error <0.05; lowercase for comparisons between chronic salt stress comparisons, upper case for temporary salt stress).


biomass. These two species, together with S. lacustris ssp. lacustris, showed a significant

increase in the ratio of aboveground to belowground biomass with increasing salinity.

In the temporary salt stress treatments (Table 3), comparable trends were observed.

Both T. latifolia and S. lacustris ssp. lacustris showed a significantly depressed final total

biomass in the three week, 18% S treatment 7. Under these conditions, their ratio of

aboveground to belowground biomass appeared significantly increased. These effects

were not found for P. australis and S. maritimus. The other short salt stress treatments had

no significant effects on the final total biomass of any of the four species.

The mean number of tubers per plant and per belowground biomass of S. maritimus

only showed significant effects in the chronic 18% S treatment, where a decrease in tuber

number per plant (to a value of 3.3�1.1, mean �1 se) was combined with an increased

number of tubers per g belowground biomass (to a value of 12.2�1.9). The mean values

of all other treatments taken together were 14.6�0.6 tubers per clone, and 1.3�0.1 tubers

gÿ1 belowground biomass.

3.3. Effects of chronic salt stress on biomass development and morphology

A significant interaction effect between age and treatment was found for all species on

aboveground biomass and RGR (Fig. 2). This effect can be attributed to the strongly

limited biomass development in treatment 3, and the slightly limited biomass

development in treatment 2. During the first part of the experiment, RGR values for

P. australis and S. maritimus in treatment 3 were significantly depressed compared to

their control RGR. Subsequently, although the salinity levels were not changed, the

treatment RGR values recovered back to the control values in all cases. Nevertheless, the

aboveground biomass levels often remained influenced: for almost all dates and species,

treatment 3 biomass differed significantly from that in the other two treatments.

T. latifolia also showed a significant difference between the control and treatment 2 from

the 5th measurement date onwards.

Comparable observations were made for main shoot length, number of leaves on the

main shoot and number of new vertical shoots (Table 3). During the experimental period,

RGR values for these morphological characteristics (not shown) at first remained very

low or showed a serious decline for treatment 3, and to a lesser extent even for treatment

2. They subsequently recovered to control values, or the plants died. For treatment 2, a

significantly reduced length was found in T. latifolia and P. australis, but both Scirpus

species were hardly affected. Its only effect on leaf number was found in T. latifolia (a

slight reduction) while shoot number was reduced only in S. lacustris ssp. lacustris.

T. latifolia hardly made new vertical shoots during the experiment in any treatment.

Treatment 3 killed T. latifolia and S. lacustris ssp. lacustris; length, leaf number and

number of vertical shoots in the other two species declined by 30±80%.

3.4. Effects of temporary salt stress on biomass development and morphology

For aboveground biomass of T. latifolia and S. lacustris ssp. lacustris, a two-way

ANOVA showed a significant interaction effect of age and treatment. The other two

species showed a significant effect of age and treatment (see Figs. 3 and 4). Regarding


Fig. 2. Development of the estimated aboveground biomass (left; g DW potÿ1) and its relative growth rate

(right; g gÿ1 DW) under chronic salt stress. Treatments: 1�control, 2�1.8% S, 3�18% S.


Fig. 3. Development of the estimated aboveground biomass under temporary salt stress (g DW potÿ1). one week

(left) and three weeks (right) exposure (periods in between vertical lines). Treatments: 1 ±control, 4 ± 1.8% S,

one week; 6 ± 1.8% S, 3 weeks; 5 ± 18% S, 1 week; 7 ± 18% S, 3 weeks.


Fig. 4. Effect of one week (left) and three weeks (right) of temporary salt stress (period in between vertical

lines) on RGR values of the estimated aboveground biomass (g gÿ1 DW). Treatments: 1�control, 4�1.8% S,

one week; 6�1.8% S, three weeks; 5�18% S, one week; 7�18% S, three weeks.


their RGR, all species had a significant interaction between age and treatment, which was

predominantly due to the results in the three weeks salt stress treatments (see Fig. 4).

In general, the one week salt stress treatments did not strongly affect aboveground

biomass development: significant differences in biomass were only found for T. latifolia

between the control and treatment 5 (one week 18% S) during the first three

measurements after the end of the salt stress. The RGR values for these treatments did not

show any significant differences.

In all the species, aboveground biomass was most strongly influenced by treatment 7

(three weeks 18% S, Fig. 3), although there also appeared to be a small but consistent

difference between the control and treatments 4 and 6 in P. australis and between the

control and treatment 6 in S. maritimus. Only a few significant differences in biomass

could be detected: for T. latifolia in treatment 7 they were present right from the start of

the salt stress till the end of the experiment despite a clear tendency to recover.

Furthermore, S. lacustris ssp. lacustris showed a low value at the end of this high salinity

treatment, which disappeared subsequently. An interesting phenomenon is the

significantly increased RGR value after the end of three weeks, 18% S treatment in all

species but T. latifolia. This resulted in a fairly strong increase in biomass, enabling these

species to reach the same biomass levels as the control.

Plant morphology at the date of harvest of the temporary salt stress treatments

(Table 3) indicated that the main shoot length was depressed in T. latifolia by treatment 5

and 7 (one or three weeks of 18% S, respectively); and it recovered only in the former

treatment. P. australis showed a reduced length in treatment 7, but recovered fully. These

decreasing lengths were due to mortality of the uppermost leaves. Both Scirpus species

showed a slightly declining trend in length for all treatments; in treatment 7, length of

S. lacustris decreased most rapidly, but plants in the other treatments eventually reached

the same value. All species showed a significant reduction in leaf number of the main

shoot in treatments 5 and 7. Only P. australis remained slightly below the control level

after treatment 7; for the same treatment, T. latifolia showed a remarkable full recovery

after a severe decline. In all species except T. latifolia, the shoot number showed a

significantly increased RGR after treatment 7.

4. Discussion

4.1. Effects of chronic and temporary salt stress

From the chronic salt stress experiment, it was apparent that the constant high salinity

level used (18% S) left no possibilities for T. latifolia and S. lacustris ssp. lacustris to

survive. Although P. australis and S. maritimus managed to stay alive, they could only do

so at a very low biomass level making them vulnerable for disturbances like water level

changes (Weisner and Ekstam, 1993; Coops et al., 1994). Furthermore, they hardly

developed new shoots. Clearly, at the high salt levels (above 140 g Clÿ1 lÿ1 in the upper

few cm) observed in the exposed shoreline of Lake Volkerak-Zoom (see Slager, 1996),

none of the four species had much potential for the creation of a belt of emergent

vegetation. This confirms the validity of the hypothesis that the permanently high salinity


of the supralittoral shoreline sediments of Lake Volkerak-Zoom is the main reason for the

poor development of emergent vegetation (Vulink and Coops, 1995). In contrast, at the

constant low salinity level of 1.8% S (treatment 2), all species apart of T. latifolia

appeared more or less promising for establishment of a shoreline vegetation from

seedlings.

All species apart from T. latifolia showed a short, significant increase in aboveground

biomass RGR after the three weeks, 18% salt stress was lifted. This enabled them to grow

back fairly quickly to the control aboveground biomass levels. Possibly, the response

could be due to re-use of carbohydrates that were accumulated to create a high osmotic

pressure during the salt stress period. A reduction in nutrient availability due to the

necessary flushing associated with this treatment seems a much less likely explanation.

The salt stress experiments were carried out under permanently waterlogged sediment

conditions and thus it cannot be decided whether a combined effect of an increasing

salinity, lowering of the water table and high summer temperatures could lead to other

conclusions. We assume that under more extreme conditions, the differences in salinity

tolerance between the species would not change much, although probably all species

would show a more severe growth depression.

It must be realized that we have studied the response of plants grown from seedlings. It

is known for all these species that propagation and survival, at least in the short term, are

governed more by successful vegetative reproduction (by means of rhizomes and/or

tubers), than through seeds. In general, seed germination and/or seedling establishment

occur at rather low rates for the species under study (Ter Heerdt, 1995). The capacity of

the plants grown from rhizomes to survive adverse conditions like (temporary) salt stress

may differ considerably from that of seedlings; see for instance Lissner and Schierup

(1997) for P. australis. Hellings and Gallagher (1992) grew P. australis from rhizomes for

8 months at 15 and 30% S. They found a strong reduction in plant height, shoot density

and biomass, but not as dramatic as observed in this study. Thus, the present results are

applicable to situations in which colonization and establishment from seeds are the most

important key factors determining success, i.e. in the first season of a new vegetation.

Comparisons between our findings and other studies are relatively difficult. The

species under study are known to show quite some phenotypic plasticity, which is

certainly partly linked to genotypic variation. Different `strains', `ecotypes', `varieties' or

even `subspecies', responding differently to the same conditions, are known in literature

(see for instance Van der Toorn (1972) for P. australis and Clevering (1995) for Scirpus).

Still, the above findings are in reasonable agreement with general knowledge about salt

tolerance in these four species (Haslam (1971) and Van der Toorn (1972) for P. australis;

Shay and Shay (1986) and Weeda et al. (1994) for T. latifolia and S. lacustris ssp.

lacustris; Flowers Dietert and Shontz (1978); Liefers and Shay (1980) and Hutchinson

(1982) for S. maritimus).

In our experiments, S. maritimus showed a negative relation between salinity and tuber

number. In contrast, Mercado et al. (1971) found that S. maritimus responded to

increasing salinities with an increased number of shoots and tubers. The highest salinity

used by them was about 11.5% S. Our plants at 18% S did show an increased tuber

number per unit belowground biomass, but the implications of such an increase for

survival are limited when the belowground biomass development is hampered. If this


response also would exist under less saline conditions, it could assist in speeding up

establishment of this species.

In conclusion, we recommend further work aiming at the creation of helophyte belts in

Lake Volkerak-Zoom to concentrate on P. australis and S. maritimus. These species

showed the best development both under chronic and temporary salt stress.

Acknowledgements

We thank Prof. Dr. Patrick Denny, Dr. Jan Vermaat (IHE), Dr. Hugo Coops (RIZA) and

an unknown referee for their comments on the manuscript. Mr. Frans Kerkum (RIZA)

provided germinated seeds of the four helophyte species and collected the necessary

sediment from Lake Volkerak-Zoom. This work was financed by the Institute for Inland

Water Management and Wastewater Treatment (RIZA, project no. 64.097286); we thank

RIZA for allowing us to use the data for publication.

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Four helophyte species growing under salt stress: their salt of life

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